l. Field of the Invention
This invention relates broadly to the agglomeration of finely-divided solid particulate material, and in particular is directed to a new and improved method for the agglomeration of certain flue dusts and similar materials from furnaces and the like which heretofore have been considered prohibitively difficult or expensive to agglomerate.
In many industrial and commercial operations it is desirable to agglomerate finely-divided particulate material, typically soluble to a substantial degree in water, into particles of larger size for further processing or for convenience in disposal. For example, in the manufacture of pellets and tablets for animal feed and pharmaceutical and other uses it is customary first to agglomerate the finely-divided or powdered and essentially water-soluble ingredients into granules, and then to compress or extrude those granules into tablets or pellets. Typically, such agglomeration or granulation is carried out batch-wise, in large containers, with the addition of water or other liquids and the aid of rotating impellers.
In other situations where it also would be desirable to agglomerate a finely-divided particulate material for convenience in disposal or for further processing, it heretofore has not been considered feasible to do so with conventional agglomeration techniques and apparatus. In many instances this is because the particulate material is essentially non-soluble or only very sparingly soluble in common liquids such as water. An example is the particulate matter exiting as flue dusts from steel-making furnaces and the like, which is extremely fine in particle size, and for that reason, difficult to handle or dispose of in its original condition. Such flue dusts oftentimes contain valuable constituents, such as derivatives of zinc and the like in significant concentrations, which it would be desirable to recover. Unfortunately, because of the need to agglomerate such dusts in order to process them for the recovery of their valuable ingredients, and because of the difficulty or expense involved in agglomerating them with conventional methods and apparatus, the recovery of such valuable ingredients heretofore frequently has not been technically and economically feasible.
2. Description of the Prior Art
One conventional process for the recovery of a metal such as zinc from waste materials involves the introduction of the zinc-bearing waste material into an inclined, rotating furnace or kiln, in which the zinc derivative is first reduced to elemental state by contact with carbonaceous material at elevated temperatures and then vaporized by heat from a countercurrent burning gas stream which conveys the zinc in vapor state out of the kiln for further processing. In order that the feed waste material itself not be blown out of the kiln, it must be of a minimum particle size. Thus, flue dusts such as the zinc-containing dusts previously referred to can not satisfactorily be processed in the kiln except through the agglomeration of the particulate waste material before it is introduced into the kiln. Such agglomeration, or pelletizing, heretofore has been achievable by spraying the finely-divided particulate material together with water onto a rapidly rotating circular disc or plate, and collecting the agglomerated particles that are discharged from the rotating plate. While satisfactory to a degree in terms of the agglomerated product, this latter method is too time-consuming and expensive to be considered generally suitable for the agglomeration of flue dusts such as zinc-containing dusts from electric steel-making furnaces.